The invention relates to a thrust piston pump for an active substance dispenser for the reproducible discharge more particularly of pharmaceutical products and the like, with a pump piston guided in a pump cylinder from a starting position via a pump stroke, the associated pump chamber of the piston being connected on the one hand to a suction duct arranged in an active substance reservoir for the suction of active substance and on the other hand to a discharge duct and with which is associated a second piston moving therewith and whose associated pressure chamber is connected to a duct, the latter being closable by a valve opening in differential pressure-dependent manner.
In a thrust piston pump known from a German Austigiselift No. 1 290 043, the second piston in the form of a control piston which is smaller than the pump piston and therefore is in advance during the piston stroke, serves to open a delivery valve provided on the pump piston after covering a predetermined pump stroke distance. However, at the instant of opening of this delivery valve the overpressure in the pump chamber, which is in any case smaller than a pump chamber which does not increase in size with the pump stroke and which keeps the control piston in the valve-opening position suddenly drops, so that the delivery valve is immediately closed again and cannot ensure an adequate active substance discharge. Moreover, in this arrangement, apart from the delivery valve, it is also necessary to have a valve opening on the inlet side in the feed direction, so that a relatively complicated pump construction is obtained.